The estrogen receptor (ER) plays an important role in the development of the normal breast and in breast cancer. The goal of this project is to build on our progress in defining the critical roles of coactivators and corepressors in ER action to better define their role in the development of the normal mammary gland and in breast cancer. The gene encoding AIB1 was cloned both as an ER coactivator and as a gene Amplified In Breast cancer, raising the possibility that it is a breast cancer oncogene. In the prior funding period we developed a transgenic mouse model in which AIB1 is over-expressed in the mammary gland and showed that AIB1, when over-expressed, is capable of inducing mammary hyperplasia and cancer. The central focus of this proposal is to determine the mechanism by which AIB1 acts as a mediator of estrogen signaling and as an oncogene in the breast. We will test these hypotheses: (1) the phenotype induced by AIB1 over-expression is autonomous to the mammary epithelium; (2) cyclin D1 is a critical mediator of the action of AIB1 in the mammary epithelium; (3) cross-talk between growth factor signaling pathways and AIB1 plays an important role in modulating ER activity in the mammary gland; and (4) the newly discovered role of AIB1 in determining the stability of the ER is critical for its ability to modulate ER activity. We will directly test these hypotheses in normal mammary epithelial cells, breast cancer cell lines, and transgenic mice. These studies will be significantly facilitated through important interactions with the Weinberg, Sicinski, Brugge, Ewen and Livingston Labs. Taken together these studies will provide a better understanding of the role played by alterations in estrogen signaling in breast cancer. The identification of the factors which are the critical regulators of the alterations in estrogen signaling will become new therapeutic targets and may lead to the development of improved strategies for the prevention and treatment of breast cancer.